In general, a cordless electric tool uses a secondary battery pack that is rechargeable with a charging device as a power supply. A lithium ion (Li-ion) battery cell is commonly used to form the secondary battery pack. A charging device for a Li-ion type battery pack generally charges the battery pack with a constant charging current and at a constant charging voltage. In particular, in order to avoid reverse charging for each battery cell, the charging device charges the battery pack with the constant charging current until the battery voltage reaches a predetermined value, for example, 4.2 V/cell, and then charges at the constant charging voltage until the current passing through the battery pack reduces under a predetermined value due to full charging.
There are two types of battery pack to be charged by the charging device: a first type of battery pack and a second type of battery pack. The first type of battery pack is a 4S1P type of battery pack having a nominal voltage of 14.4 V, in which four battery cells are connected in series. The second type of battery pack is a 4S2P type of battery pack having a nominal voltage of 14.4 V, in which a pair of battery cells is connected in parallel and four pairs of parallel-connected battery cells are connected in series. Each of the battery packs generally has an identifying resistor identifying the configuration of the battery cells in the battery pack.
If one charging device selectively charges the above two different types of battery packs and flows the same amount of charging current through each battery pack, the amount of current passing through each cell of the 4S1P type is twice as much as the amount of current passing through each cell of the 4S2P type. This phenomenon may result in shortening the lifespan of the 4S1P type, compared with the lifespan of the 4S2P type.
The charging device has a unit for detecting the identifying resistor of the battery pack to determine the type thereof. When the first battery pack having a nominal voltage of 14.4 V (which includes four battery cells connected in series) has a resister having a resistance of a, and the second battery pack having a nominal voltage of 18 V (which includes five battery cells connected in series) includes a resister having a resistance of b, a microcomputer of the charging device detects the divided voltage of a reference voltage by the resistor of a or b and the internal resistor w of the charging device, and then determines which the first or second battery pack is connected with the charging device.
As described above, each battery pack has an identifying resistor having its own resistance. However, if the charging device erroneously determines the resistance of the identifying resistor, the charging device may feed an improper current flow to the battery pack, which may adversely affecting the lifetime of the battery pack.
An object of the invention is to provide a charging system in which a charging device suitably feeds a charging current to a battery pack, depending on the type of the battery pack.